A microprocessor is an integrated circuit (IC) built on a tiny piece of silicon. A microprocessor contains millions of transistors interconnected through fine wires made of aluminum or copper.
Microprocessor fabrication is a complex process involving many steps. Microprocessors are typically built by layering materials on top of thin rounds of silicon, called wafers, through various processes using chemicals, gases and light. In chip making, very thin layers of material, in carefully designed patterns, are put on the blank silicon wafers. The patterns are computerized designs that are miniaturized so that up to several hundred microprocessors can be put on a single wafer.
Because the patterns are so small, it is nearly impossible to deposit material exactly where it needs to be on the wafer. Instead, a layer of material is deposited or grown across an entire wafer surface. Then, the material that is not needed is removed and only the desired pattern remains.
The microprocessor fabrication process begins with “growing” an insulting layer of silicon dioxide on top of a polished wafer in a high temperature furnace. Photolithography, a process in which circuit patterns are printed on the wafer surface, is next. A temporary layer of a light sensitive material called a “photoresist” is applied to the wafer. Ultraviolet light shines through clear spaces of a stencil called a “photomask” or “mask” to expose selected areas of the photoresist. Masks are generated during a design phase and are used to define a circuit pattern on each layer of a chip. Exposure to light chemically changes the uncovered portions of the photoresist. The machine used to do this is typically called a “scanner” because it scans one die or a few die at a time, then steps to the next die or set of die until it has exposed the entire wafer.
An active area of the mask is exposed to a vacuum environment of a scanner and is at high risk of accumulating particles. If a particle lands in a critical part of the active area then it will lead to a printed defect on the wafer. This defect causes decreased yields, or increased wafer cost in rework if the defect is fortuitously caught prior to further wafer processing by wafer defect metrology.
Like reference symbols in the various drawings indicate like elements.